As is known, a user authentication by a fingerprint recognition is not only convenient to use but also excellent in terms of a security and cost-effectiveness and is broadly commercialized now. A device for acquiring a fingerprint image is generally classified into a semiconductor type device using semiconductor touch sensors and an optical device using optical sensors. Of these devices, an optical sensor has a strong durability (a scratch resistance, chemical resistance, abrasion resistance, and shock resistance) and exhibits a high recognition rate due to the visibility of an image (an image resolution), and thus the optical device is more preferred.
Among optical fingerprint recognition apparatus employing the optical sensor, an absorbent optical fingerprint recognition apparatus has a configuration in which an incident light on a portion with which a fingerprint does not come in contact is totally reflected from a surface of a prism and an incident light on ridges is refracted from or absorbed in moisture or oil, thereby resulting in condensing on the lens only a very small amount of light. As a result, in principle, the ridges portions have got filmed in a dark image, and the portions of the valleys or non-contact portions of a fingerprint have got filmed in a bright image. According to this basic principle, when the ridge portions of the fingerprint being confronted with the surface of the prism is dry, the contact area becomes reduced and medium such as moisture, oil or the others to refract and absorb the light also becomes lessened. As a result, the absorption optical fingerprint recognition apparatus has a disadvantage that the amount of light in total reflection increases and thus the fingerprint image that is filmed is unclear or blurred. In other words, an absorption optical system is vulnerable to a dry fingerprint and thus a dispersive optical system began to be used as a way to overcome these shortcomings.
The dispersive optical system acts in opposition in comparison with the absorption optical system, in principle. The dispersive optical system includes a material such as a black lacquer that absorbs light, such as a black lacquer, on one side of a prism, and condenses on lens the light scattered at the ridges portions of the fingerprint contacted thereto. As a result, as opposed to the absorption optical system, the dispersive optical system exhibits a property that the ridge portions is filmed in a bright image and the valley portions or non-contacted portions with the fingerprint are filmed in a dark image.
However, despite of the use of the dispersive optical system, it does not solve the fundamental vulnerability to the dry fingerprint, but it merely improves a portion of the characteristic of the dry fingerprint.
On the other hand, an AGC (Adaptive Gain Control) has been introduced to improve the vulnerability to the dry fingerprint.
The AGC is a technique to adjust again which represents an exposure time and the amplification ratio of camera sensors, and collectively refers to a process looking for an optimal fingerprint image while reducing the exposure value of a camera when it is judged that an image which is captured is not suitable for a fingerprint recognition.
However, the AGC has two main drawbacks.
First, it takes a long time to capture a fingerprint image, and the state (shape) of the fingerprint continues to change in the course of feedback. This is caused by the characteristics of the AGC to determine the suitability for the captured image, apply a new exposure value to the captured image based on the determination, and perform cyclically repeatedly the determination and analysis on an image which has been changed again until an optimal exposure value is obtained. As such, the AGC is to acquire a dry fingerprint image with a better quality by raising the contrast between images through a feedback process.
Second, a reference background image (background image without a fingerprint) for a background processing to be subjected to an optimal image processing is obtained under a normal exposure. Accordingly, there occurs a problem that the reference background image is no longer used when the exposure is changed arbitrarily. Moreover, in the above case, because a fingerprint to be captured, i.e., a finger has already been put on an optical system when the AGC is being executed, it does not newly obtain a blank image needed for the background processing.